1. Field of the Invention
This invention relates to a method for burning pulverous alkali-containing raw materials such as cement raw meal, and a plant for practicing the inventive method.
2. Description of the Prior Art
For the sake of convenience the term "impurities" used hereinafter should be understood as comprising not only alkalis but also chlorine and sulphur.
When a preheater is constituted by one string receiving preheating air in the form of waste gas from the kiln and a second string receiving preheating gas through the calciner, impurities in the form of dust or coatings on the material being treated will accumulate in the preheater stages and consequently be liable to create blockages in the cyclones and riser pipes in a critical temperature range. This is particularly so in the kiln string in which the impurity concentrations will be much more critical than in the calciner string due to the volatile impurities released in the kiln.
The reason why blockages are formed is to be found in the fact that when the temperature of the raw materials increases down the cyclones and kiln, the content of alkalis, sulphur and chlorine compounds vaporizes and is drawn with the kiln gases up the cyclones in countercurrent to the raw materials, and when reaching lower temperatures the volatile impurities are liable to condense and solidify on cold surfaces in the preheater or on the raw material particles passing by.
In the condensing phase the impurities cause part of the raw materials to adhere thereto and total blockage will very quickly develop.
It is known to direct part of the kiln gases to the precipitator, by-passing the preheater in order to reduce the impurity content, which would otherwise accumulate. However, there is still a great risk of blocking up the preheater, as the plant preferably should be run with a minimum of by-pass to meet the requirements of economical operation.
The easiest but most expensive way to overcome impurity troubles in the cyclones is to dispose of all of the kiln gases and only to use spent cooling air to preheat the raw material passing down the cyclone preheater. I have invented an improved method of reducing the impurity concentrations in the cyclones and riser pipes, and consequently the risk of blockages.